Cloud reflectivity modification

refer to caption and image description
The exhaust from ships already causes more and brighter clouds above the oceans.

Cloud reflectivity modification is a climate engineering technique (or more specifically, a solar radiation management technique) that would seek to reflect a very small fraction of sunlight back into space in order to offset many of the negative impacts of climate change.[1] In particular, it would seek to alter clouds to make them more or less reflective, in order to change their effect on climate.

There are two hypothesised forms: cirrus thinning and marine cloud brightening. High, cold cirrus clouds tend to allow sunlight through, yet trap infra-red (or longwave) radiation from Earth. Therefore, thinning or removing these clouds reduces their longwave trapping capacity and results in a cooling effect on Earth's climate.[2]

Conversely, low, warm clouds tend to be highly reflective to sunlight, especially when the cloud is bright due to the presence of smaller droplets (because of the Twomey effect). Therefore, modifying these clouds to make them more reflective cools the climate. This proposed technique is known as 'marine cloud brightening', 'marine sky brightening'[3] or 'cloud whitening' of low cloud.

By modifying the reflectivity of clouds, the albedo of Earth would be altered. The intention is that this technique, in combination with greenhouse gas emissions reduction (and possibly other climate engineering techniques) will be sufficient to control global warming. The effect is expected to be fully reversible, as the cloud condensation nuclei particles precipitate naturally. However, like any planetary-scale project dealing with the complex climate system, there is a non-trivial risk of unintended consequences.

Basic principles

Low level stratocumulus clouds cover approximately one-third of the oceans surface. These clouds possess Albedo levels of 0.3-0.7.[4] The levels of albedo present in the low-level marine stratocumulus clouds create a significant global cooling effect.[5] Theoretically, by increasing the albedo of the clouds, via spraying sea water(NaCl) droplets approximately 1 μm in size, the cooling effects of the clouds are increased and potentially their longevity is enhanced.[6] By creating cloud condensation nuclei, it might be possible to change the albedo of clouds to make them appear whiter.[7] This can be done using a variety of chemicals and techniques, with seawater sprays from ships being a simple example. Broadly speaking, the marine environment has a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea, so this technique is more effective over the ocean than over land.

Models

HadGAM1 general circulation model

A study by Jones, Latham, and Smith using the Hadley Center's General Circulation Model, suggests that warming due to a x2 increase in CO2 could potentially be compensated for by a doubling of the droplet concentration number in low-level marine stratocumulus clouds, accounting for anthropogenic aerosol production, in three regions—off the coasts of the Americas and West Africa. The three regions combined cover approximately 3% of Earth's surface.[8]

Model of marine stratocumulus clouds developed by Bower, Jones and Choularton

Bower, Jones and Choularton created a model to analyze the effectiveness of albedo modification on clouds.[9] A 2006 study simplified the model.[10] Their model illustrates the practicality of the technique. It demonstrates droplet size is not of that much importance, that location of clouds is of limited importance, and that significant cooling can be achieved with a level of .03.[10]

Proposed schemes

Seawater spray

Various schemes have been suggested,[11][12][13] such as that proposed by John Latham and Stephen Salter,[14][15] which works by spraying seawater in the atmosphere to increase the reflectiveness of clouds. The extra condensation nuclei created by the spray will change the size distribution of the drops in existing clouds to make them whiter.[16] The sprayers would use a fleet of around 1500 unmanned Rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth.[11][17] The whitening is achieved as a result of the Twomey effect. In order to significantly cool Earth, the vessels have to spray sea water droplets at a rate of 50 cubic meter per second over a large portion of Earth's ocean surface.[18]

This technique can give >3.7 W/m2 of globally averaged negative forcing,[17][19] which is sufficient to reverse the warming effect of a doubling of CO2.

Ocean foams

Creation of ocean foams has been suggested. When bubbles in the foams burst, they loft small droplets of seawater, which can modify the albedo of clouds.[20]

Ultrasonic excitation of a liquid using a piezo-electric transducer

This technique works by creating faraday waves at a free surface. If the waves are steep enough the droplets of sea water will be thrown from the crests and particles can enter into the clouds within a predictable area. However, a significant amount of energy is required.[21]

Electrostatic atomisation of seawater drops

This technique utilizes mobile spray platforms which move to adjust to changing weather conditions. A proposed idea is to use unmanned ships that shoot sea spray into the air.[22]

Cloud seeding yachts

Stephen Salter developed an unmanned yacht that uses Flettner rotors for propulsion. The rotors are vertical cylinders located on the deck of the yacht. They spray the tiny water droplets into the low level clouds to enhance their reflectivity. The power for the rotors and the ship is generated from underwater turbines. Approximately 1,500 of these ships would be required for the scheme to effectively cool the planet.[23] The Royal Society states that "approximately 1500 spray vessels would be required to produce a negative forcing of -3.7W m^-2. Each vessel would require approximately 150 kW of electrical energy to atomize and disseminate seawater at the necessary continuous rate (as well as to support navigation, controls, communications, etc.), so that the global power requirement is approximately 2.3x10^8 Watts." [24] This technique requires far less energy compared to many other geoengineering techniques.

Ocean sulfur cycle enhancement

Main article: sulfur cycle

Enhancing the natural sulfur cycle in the Southern Ocean by fertilizing a small portion with iron in order to enhance dimethyl sulfide production and cloud reflectivity.[25] The goal is to slow Antarctic ice from melting and raising sea level[26] Such techniques also tend to sequester carbon, but in this specific project the enhancement of cloud albedo was both the desired outcome and measured result.[27]

This technique can give only 0.016 W/m2 of globally averaged negative forcing,[19] which is practically insignificant as a contribution to reducing global warming. However, as it is a regionally acting technique its effects are concentrated in an influence on the climate of Antarctica.

Advantages and disadvantages

Cloud reflectivity modification appears to have most of the advantages and disadvantages of solar radiation management in general. Some advantages and disadvantages are specific to it, relative to other proposed solar radiation management techniques.

Advantages

Marine cloud brightening possesses the general advantages of solar radiation management. For example, it presently appears to be inexpensive relative to suffering climate change damages and greenhouse gas emissions abatement, fast acting, and reversible in its direct climatic effects. Compared with other proposed solar radiation management methods, such as stratospheric aerosols injection, marine cloud brightening may be able to be partially localized in its intended effects.[28] This could, for example, be used to stabilize the West Antarctic Ice Sheet. Furthermore, marine cloud brightening, as it is currently envisioned, would use sea water and wind, which are both natural and plentiful.

Disadvantages

Further research

Cirrus Cloud Thinning

Synoptic cirrus clouds may be susceptible to modification to reduce their lifetime and optical thickness and hence their net positive radiative forcing (opposite to the low-cloud scheme), using seeding mechanisms distributed by airliners.[30] It is believed that the synoptic cirrus in the high latitude upper troposphere are formed by homogeneous nucleation, resulting in large numbers of small ice crystals. If ice nuclei are introduced into this environment, the cirrus may instead form by heterogeneous nucleation. If the concentration of ice nuclei is tuned such that the resulting cloud particle density is less than for the natural case, the cloud particles should grow larger due to less water vapor competition and attain higher settling velocities. The net effect would be a reduced optical thickness and a reduced cloud lifetime allowing more infra-red radiation to be emitted at the top of the atmosphere. Less infra-red radiation in the atmosphere would cool the climate.

See also

References

  1. ↑ "The Royal Society" (PDF). royalsociety.org. Retrieved 2015-11-03.
  2. ↑ Muri, H., J. E. Kristjánsson, T. Storelvmo, and M. A. Pfeffer (2014), The climatic effects of modifying cirrus clouds in aa climate engineering framework, J. Geophys. Res. Atmos., 119, 4174–4191, doi:10.1002/2013JD021063.
  3. ↑ Schäfer et al., (2015). The European Transdisciplinary Assessment of Climate Engineering (EuTRACE): Removing Greenhouse Gases from the Atmosphere and Reflecting Sunlight away from Earth. Funded by the European Union’s Seventh Framework Programme under Grant Agreement 306993.
  4. ↑ Schwartz and Slingo, 1996 S.E. Schwartz and A. Slingo In: P. Crutzen, Editor, Clouds, Chemistry and Climate: Proceedings of NATO Advanced Research Workshop (1996).
  5. ↑ Latham, J. (2002). "Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds". Atmospheric Science Letters 3 (2–4): 52. Bibcode:2002AtScL...3...52L. doi:10.1006/asle.2002.0048.
  6. ↑ Computational Assessment Of A Proposed Technique For Global Warming Mitigation Via Albedo-enhancement Of Marine Stratocumulus Clouds K Bower - T Choularton - J Latham - J Sahraei - S Salter - Atmospheric Research - 2006.
  7. ↑ Twomey, S. (1977). "The Influence of Pollution on the Shortwave Albedo of Clouds" (PDF). J. Atmos. Sci. 34: 1149–1152. Bibcode:1977JAtS...34.1149T. doi:10.1175/1520-0469(1977)034<1149:TIOPOT>2.0.CO;2.
  8. ↑ Jones, A., Latham, J., Smith, M.H., submitted for publication. Radiative forcing due to the modification of marine stratocumulus clouds, Atmos. Sci. Letters.
  9. ↑ Bower, K.N., Jones, A., and Choularton, T.W., 1999. A modeling study of aerosol processing by stratocumulus clouds and its impact on GCM parameterisations of cloud and aerosol. Atmospheric Research, Vol. 50, Nos. 3–4, The Great Dun Fell Experiment, 1995-special issue, 317–344.
  10. 1 2 Computational Assessment Of A Proposed Technique For Global Warming Mitigation Via Albedo-enhancement Of Marine Stratocumulus Clouds K Bower - T Choularton - J Latham - J Sahraei - S Salter - Atmospheric Research - 2006.
  11. 1 2 Latham, J. (1990). "Control of global warming" (PDF). Nature 347 (6291): 339–340. Bibcode:1990Natur.347..339L. doi:10.1038/347339b0.
  12. ↑ Latham, J.; Salter, S. "Preventing global warming by increasing cloud albedo" (PDF). Retrieved 2008-04-20. (A brief handout, with artist's renderings.)
  13. ↑ Keith Bower; et al. (2006). "Assessment of a Proposed Technique for Global Warming Mitigation via Albedo-Enhancement of Marine Stratocumulus Clouds". Atmospheric Research 82 (1-2): 328–336. Bibcode:2006AtmRe..82..328B. doi:10.1016/j.atmosres.2005.11.013.
  14. ↑ Latham, J. (2002). "Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds" (PDF). Atmos. Sci. Lett. 3: 52–58. Bibcode:2002AtScL...3...52L. doi:10.1006/asle.2002.0099.
  15. ↑ Salter, S, G. Sortino & J. Latham (2008). "Sea-going hardware for the cloud albedo method of reversing global warming". Phil. Trans. R. Soc. A 366 (1882): 3989–4006. Bibcode:2008RSPTA.366.3989S. doi:10.1098/rsta.2008.0136. PMID 18757273.
  16. ↑ Panel on Policy Implications of Greenhouse Warming, National Academy of Sciences, National Academy of Engineering, Institute of Medicine (1992). Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base. The National Academies Press. ISBN 0-585-03095-2.
  17. 1 2 Latham, J., P.J. Rasch, C.C.Chen, L. Kettles, A. Gadian, A. Gettelman, H. Morrison, K. Bower, T.W.Choularton (2008). "Global Temperature Stabilization via Controlled Albedo Enhancement of Low-level Maritime Clouds". Phil. Trans. Roy. Soc. A 366: 3969–87. Bibcode:2008RSPTA.366.3969L. doi:10.1098/rsta.2008.0137. PMID 18757272.
  18. ↑ BBC NEWS | Programmes | Futuristic Fleet of 'cloudseeders'" BBC News - Home. 15 Feb. 2007. 20 Nov. 2010. <http://news.bbc.co.uk/2/hi/programmes/6354759.stm>.
  19. 1 2 Lenton, T.M. and N.E. Vaughan (2009). "The radiative forcing potential of different climate geoengineering options" (PDF). Atmos. Chem. Phys. Discuss. 9: 2559–2608. doi:10.5194/acpd-9-2559-2009.
  20. ↑ Evans, J.; Stride, E.; Edirisinghe, M.; Andrews, D.; Simons, R. (2010). "Can oceanic foams limit global warming?". Climate Research 42 (2): 155–160. doi:10.3354/cr00885.
  21. ↑ Barreras et al., 2002 F. Barreras, H. Amaveda and A. Lozano, Transient high frequency ultrasonic water atomization, Exp. Fluids 33 (2002), pp. 405–413. View Record in Scopus | Cited By in Scopus (31)
  22. ↑ BBC NEWS | Programmes | Futuristic Fleet of 'cloudseeders'" BBC News - Home. 15 Feb.2007. 20 Nov. 2010.
  23. ↑ Latham, John (15 February 2007). "Futuristic fleet of 'cloudseeders'". BBC. Archived from the original on 2012-07-25. Retrieved 2012-07-25.
  24. 1 2 3 4 5 6 7 Latham, John; Rasch, Philip; Chen, Chih-Chieh; Kettles, Laura; Gadian, Alan; Gettelman, Andrew; Morrison, Hugh; Bower, Keith; Choularton, Tom (2008-11-13). "Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds". Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 366 (1882): 3969–3987. doi:10.1098/rsta.2008.0137. ISSN 1364-503X. PMID 18757272.
  25. ↑ Wingenter, O.W., Elliot, S.M., Blake, DR. (2007). "New Directions: enhancing the natural sulfur cycle to slow global warming". Atmospheric Environment 41: 7373–737. Bibcode:2007AtmEn..41.7373W. doi:10.1016/j.atmosenv.2007.07.021.
  26. ↑ Coale, K. H.; Johnson, K. S.; Buesseler, K.; Sofex Group (2002). SOFeX: Southern Ocean Iron Experiments. Overview and Experimental Design. American Geophysical Union, Fall Meeting.
  27. ↑ T.S. Bates, B.K. Lamb, A. Guenther, J. Dignon and R.E. Stoiber (2004). "Sulfur emissions to the atmosphere from natural sourees". Journal of Atmospheric Chemistry 14 (1-4): 315–337. doi:10.1007/BF00115242.
  28. ↑ Latham, John; Gadian, Alan; Fournier, Jim; Parkes, Ben; Wadhams, Peter; Chen, Jack (2014). "Marine Cloud Brightening: Regional Applications". Philosophical Transactions of the Royal Society A 372 (2031): art. 20140053. doi:10.1098/rsta.2014.0053. Retrieved 26 October 2015.
  29. ↑ Latham, John; Bower, Keith; Choularton, Tom; Coe, Hugh; Connolly, Paul; Cooper, Gary; Craft, Tim; Foster, Jack; Gadian, Alan (2012-09-13). "Marine cloud brightening". Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 370 (1974): 4217–4262. doi:10.1098/rsta.2012.0086. ISSN 1364-503X. PMC 3405666. PMID 22869798.
  30. ↑ Mitchell, D. L.; Finnegan, W. (2009). "Modification of cirrus clouds to reduce global warming". Environmental Research Letters 4 (4): 045102. Bibcode:2009ERL.....4d5102M. doi:10.1088/1748-9326/4/4/045102.
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